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Elektra 0.11.0
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This overview complements the introduction in the API documentation.
A Key
consists of a name, a value and metadata. It is the atomic unit in the key database. Its main purpose is that it can be serialized to be written out to permanent storage. It can be added to several aggregates using reference counting. Putting Key
objects into other data structures of supported programming languages presents no problem.
The central data structure in Elektra is a KeySet
. It aggregates Key
objects in order to describe configuration in an easy but complete way. As the name "set" already implies every Key
in a KeySet
has a unique name. A user can iterate over the Key
objects of a KeySet
. KeySet
sorts the keys by their names. This yields a deterministic order advantage. So, independent of the appending sequence and, in particular, the number of fetches and updates, KeySet
guarantees the same order of the Key
objects. Some configuration storage systems need this property, because they cannot remember a specific order. On the other hand, any particular order can easily be introduced (See order).
On the one side backends generate or store a KeySet
object and, on the other side, elektrified applications receive and send a KeySet
object. Both sides, as well as the core in between, have the possibility to iterate, update, modify, extend and reduce the key set. Appending of new or existing Key
objects extends the key set. Otherwise it can be reduced if keys are popped out. The Key
object becomes independent of the KeySet
afterwards. The user can still change such a key or append it into another key set. The affiliation to a key set is not exclusive.
Every key in a KeySet
object has a unique name. Appending Key
objects with the same name will override the already existing Key
object.
While objects of Key
and KeySet
only reside in memory, Elektra’s third class KDB
actually provides access to the global key database. KDB
, an abbreviation of key database, is responsible for actually storing and receiving configuration. KeySet
represents the configuration when communicating with KDB
. The typical elektrified application collects its configuration by one or many calls of kdbGet()
. As soon as the program finishes its work with the KeySet
, kdbSet()
is in charge of writing all changes back to the key database.
This technique has some advantages. First, applications have full control over modifying Key
and KeySet
objects without touching the key database. Second, the decision how many KeySet
objects the application administrates is left to the application. It can choose how to split up the KeySet
objects. The main reason for this technique is that for backend development the same data structure is used, and as we will see, the borderline between application and backend development becomes blurred.
The application adapts the configuration between kdbGet()
and kdbSet()
in memory. The modifications are not only faster, they also allow large atomic configuration upgrades, robust merging of settings and handling of complicated inter-relationships between keys without problematic intermediate steps. Elektrified applications, however, should be aware of conflicts. It can happen that the key database is changed while working with a KeySet
. Then, attempts to use kdbSet()
lead to a conflict. KDB
detects such situations gracefully and lets the application decide which configuration should be used.
For details and background read more about elektra data structures. For further information see the API documentation.